This invention relates to a method for the production of tin-graphite or tin/lead-graphite layers in which at least a layer of tin is deposited electrolytically. More particularly, the invention relates also to a bath for the electrodeposition of such dispersion coatings from an aqueous solution of tin(II) and possibly lead(II) salts.
For the fabrication of multiple plug contact strips, there have been used until now predominantly precious-metal coated contact pins or knives and contact jacks, which are typically applied exclusively by electroplating. To extend the useful life in these applications, it is conventional practice to provide graphite inclusions in the precious metal coatings. For example from German patent document No. DE-PS 25 42 082, and corresponding British patent document No. GB-PS 15 34 429, a cyanide silver electrolyte and a process for the electrodeposition of silver-graphite dispersion coatings are disclosed to make electrical contact wherein the graphite serves as a solid lubricant.
For some time those skilled in the art have endeavored to replace the precious metal layers for the above stated contacts by tin or tin/lead layers. Tin layers may be applied either by hot tinplating or by means of electrolytic baths. From German patent document No. DE-OS 24 13 402, gliding tin layers for electric slide contacts on plug elements are disclosed, where the tin or tin/lead layers, in particular if applied by electroplating, are incorporated in the boundary zone of the surface by a process known as ball polishing with the addition of substances which promote sliding, such as graphite powder, which at the same time promotes work-hardening.
In the prior art, therefore, the inclusion of the graphite in the tin layer occurs only mechanically in the boundary zone to a depth of about 0.5 .mu.m, and this requires a separate additional operation. Here the shape of the parts to be treated is of importance; optimum distribution on all boundary areas of the contact cannot be achieved.
It is an object of this invention which is to be described to provide an improved method for the application of layers on a tin-graphite or respectively tin/lead-graphite basis and the respective means.